1. Heat 1 tablespoon olive oil in a large skillet over medium heat. Add the ground beef and Italian sausage, then cook until browned, breaking the meat apart as it cooks.

2. Add the diced onion and cook until softened, about 4 to 5 minutes. Stir in the garlic, Italian seasoning, dried basil, crushed red pepper flakes if using, kosher salt, and black pepper.

3. Stir in the tomato paste and cook for 1 minute. Add the marinara sauce, tomato sauce, and water, then simmer for 15 to 20 minutes until slightly thickened.

4. In a medium bowl, mix the ricotta cheese, egg, 2 cups shredded mozzarella cheese, 3/4 cup grated Parmesan cheese, and 2 tablespoons chopped parsley until smooth.

5. Preheat the oven to 375°F. Spread a thin layer of meat sauce in the bottom of a 9×13-inch baking dish.

6. Layer 3 cooked lasagna noodles over the sauce, then spread part of the ricotta mixture over the noodles and spoon meat sauce on top.

7. Repeat the layers until all ingredients are used, finishing with sauce on top. Sprinkle with the remaining 1 cup mozzarella cheese and remaining 1/4 cup Parmesan cheese.

8. Cover with foil and bake for 30 minutes. Remove the foil and bake for 15 to 20 minutes more, until bubbly and lightly golden.

9. Let the lasagna rest for 10 to 15 minutes before slicing. Garnish with 1 tablespoon chopped parsley and serve.